Due to the increase miniaturisation of semiconductor elements to be fabricated so as to make smaller structures and in increasing densities, challenges are ever present in this field of technology. Photolithography of patterns that are increasingly smaller and denser on a polysilicon layer that has a relatively high reflectivity pose the problem of forming patterns wherein certain microfine features might be broadened or narrowed as a result of the undesirable high reflection due to the presence of metals such as aluminium, tungsten and copper.
Therefore, it is often necessary to provide an anti-reflective coating (ARC) to allow a better imaging of an patterning layer. In particular, an overlying layer comprising a bottom anti-reflective coating (BARC) may be formed over the underlying polysilicon layer to reduce the reflection of light during photolithographic patterning process. As shown in FIG. 1, the photoresist (PR) is then formed as a negative of the pattern to protectively cover those areas not to be etched.
The exposed overlying layer may then be etched with a recipe known for selectivity to said layer, in this case BARC, to expose the underlying layer, in this case SiN, not covered by the photoresist. The SiN layer may then be etched to form the pattern on the silicon substrate. The etching rate and depth of etch reached may be monitored by endpoint detection (EPD) systems. The etch endpoint is identified by monitoring the magnitude or intensity of an optical emission. Endpoint detection may be conducted with an optical emission spectroscopy (OES) system to detect when an upper layer (e.g. a dielectric layer), which is being etched, has been penetrated to reach the underlying layer (e.g. a polysilicon layer). Upon the etch reaching the underlying layer, the underlying material that is released into the chamber atmosphere has a signature wavelength that may be detected by the endpoint detection system.
Materials chosen as BARC depends on the wavelength used for the photo-masking process but typically include titanium nitride, silicon oxynitride (SiOxNy), silicon nitride, silicon dioxide and organic ARC materials. BARC on SiN (hereinafter “BARC/SiN”) stacks is unfortunately very common nowadays, especially for the process that called in situ trench etch because both BARC and SiN when etched releases very similar by-products or residues into the chamber atmosphere and the conventional EPD systems could not detect accurately and efficiently whether BARC is being etched or it has been completely etched and has reached the SiN layer.
This poses a problem for EPD systems using optical emission spectroscopy (OES) for all layers comprising material A overlying material B where A and B have similar or very similar endpoint wavelengths to be detected. This is because the by-products of material B released would have similar endpoint wavelength as that of material A.
U.S. Pat. No. 6,009,888 (Ye, et. al., granted to Chartered Semiconductor) disclosed the successful etching, among others, of layers of photoresist, BARC and polymer over silicon nitride after a dry etch using a combination of acid (S2O82−/HCl/H2O) bath and simultaneous UV laser irradiation to achieve an etching synergy (compares to separate bath and irradiation) without relying on endpoint detection means.
U.S. Pat. No. 6,277,716 (Chhagan, et. al., also granted to Chartered Semiconductor) includes an example which comprises layers of photoresist, BARC and nitrogen rich silicon (e.g. SiN) which includes an additional layer for endpoint detection purposes. The endpoint detect layer is formed of a nitride rich polysilicon layer and formed by depositing polysilicon in an ammonia ambient in a low pressure chemical vapour deposition (LPCVD) process. Accordingly, this requires separate and additional etch steps in the process to remove the endpoint etch layer and the BARC prior to pattern etching.
U.S. Pat. No. 6,368,975 (Balasubramhanya, et. al.; granted to Applied Materials, Inc.) discloses a method for measuring or monitoring correlated attributes of a process (e.g. electromagnetic emissions of plasma) and using Principal Component Analysis (PCA) to analyse the correlated attributes so that process event information may be obtained, including endpoint detection or “breakthrough” etching. Amidst the breath of scope and methodological sophistication, the time element or factor, e.g. the timing of an endpoint, remains a critical element of the method. There is no attempt to differentiate the closely similar endpoint wavelengths of the overlying and underlying materials.
US-2003/0043383 (Usui, et. al.) disclosed a methodology for determining endpoint which requires simultaneous multiple wavelengths to produce interference light waveforms so that, based on the time-differential waveforms, the patterns representing wavelength dependencies of the interference waveform differential values, the film thickness may be measured. Apart from having to rely on interference of light, the analysis of differential values is also time-based.